The present invention relates to a process for increasing the chemical resistance of parts which have been electroplated with zinc followed by a chromate coating, especially steel parts for use in the automotive industry. More particularly, the present invention relates to an improved process for sealing chromate conversion coatings on electrodeposited zinc, thereby increasing the chemical resistance of the zinc plated parts.
In recent years, the automotive industry has required an ever increasing degree of protection against corrosion of parts which have been electroplated with zinc and then coated with a yellow, black, white or green chromate. This need of increased corrosion protection is particularly important for zinc plated parts which are in the automobile engine compartment and thus, continually subjected to high temperatures. When such parts have been treated with conventional chromate coatings, these high temperatures cause the layer of coating, which normally contains Cr(OH).sub.3 and --CrOH--CrO.sub.4 --H.sub.2 O, to lose its water of crystallization, thereby causing a significant reduction in the chemical resistance of the coating. Typically, when such parts are subjected to temperatures of about 120.degree. C. for only two (2) hours, their resistance to corrosion, as measured by the saline fog test (ASTM B117, 5% neutral sodium chloride) is only about 40 or 50 hours. For present automotive requirements, such results are unacceptably low by a factor of at least 10.
In an attempt to improve the corrosion resistance of such zinc plated/chromated parts, different approaches have been explored. For example, U.S. Pat. No. 4,800,134, discloses a process for producing a a steel-clad roll having high chemical resistance. In this process, the steel substrate is electroplated to form a base layer of a zinc or zinc alloy matrix. To this base layer is applied a layer of particles of water insoluble chromate combined with colloidal particle or additional fines of SiO.sub.2, TiO.sub.2, Cr.sub.2 O.sub.3, Al.sub.2 O.sub.3, ZrO.sub.2, SnO.sub.2 and/or SbO.sub.5. Thereafter, an additional electroplated coating is formed which contains zinc, iron, cobalt, and/or manganese, and this coating is followed by a layer of an organic resin coating and/or an additional layer of electroplated coating. Although, the coated steel substrate produced by this process has high chemical resistance, the number of steps required in the process make it economically unattractive. Additionally, the use of colloidal particles often causes difficulties in obtaining uniform coating layers.
In European Patent Application No. 86307929.9, a process is described for improving the chemical resistance of a zinc or cadmium plated metal article. In this process, the zinc or cadmium plated part is coated with a chromate solution to form a yellow to matt olive chromate coating. Thereafter, the conversion coated article is immersed in a silicate solution for a period of time sufficient to produce an acceptable white-gray colored coating on the surface. Although this process does provide some increase in the chemical/corrosion resistance of the coating, the corrosion resistance obtained is still unacceptably low for present automotive requirements.
In spite of the efforst which have been expended, the object of producing, economically, a zinc plated/chromate conversion coated steel substrate having high chemical/corrosion resistance has not been achieved.